Heretofore, alloyed hot-dip galvanized steel sheets have been used mainly in the automotive field. However, in the plating layer of the alloyed hot-dip galvanized steel sheet, a Zn—Fe alloy layer, which is inferior in the ductility to the underlying steel sheet, is present. In a case where a tensile strength of 980 MPa or more is required, this layer has been considered to be a problem, because the plating (or coating) adhesion thereof is poor and the plating is liable to be separated from the interface between the plating and the underlying steel sheet, at the time of working such as press molding, to thereby readily cause an appearance failure such as indentation mark.
Therefore, Patent Document 1 describes, as a measure for improving the plating adhesion, a method of enhancing the plating adhesion by a so-called anchor effect, that is, by increasing the unevenness at the interface between the coating and the underlying steel sheet. Also, Patent Document 2 discloses that when a steel sheet is heated, and then is pickled, and after the removal of the oxide layer on the surface, is subjected to plating, to thereby improve the adhesion.
Also, Patent Document 3 discloses a high-strength high-ductility hot-dip galvanized steel sheet containing, in terms of volume fraction, from 30 to 90% of a ferrite phase, 5% or more of bainite, 10% or less of martensite, and from 5 to 30% of a retained austenite phase. Patent Document 4 discloses a high-strength cold-rolled steel sheet, where the density of dislocations contained in the steel sheet is 8×1011 (dislocations/mm2) or less, and the static/dynamic ratio (=FS2/FS1) as a ratio between a quasi-static strength (FS1) at a strain rate of 0.0067 (s−1), and a dynamic strength (FS2) at a strain rate of 1,000 (s−1) is 1.05 or more. Patent Document 5 discloses an alloyed hot-dip steel sheet comprising a base iron, and a specific plating layer disposed at least on one surface thereof, wherein plating layer does not have an η phase or ζ phase in the plating layer surface, the plating layer has a composition of Fe: from 8 to 12% and Al: from 0.05 to 0.25%, with the balance being Zn, and the Γ phase at the interface with the base iron is 1.0/μm or less.
However, in the prior art, the difference in ductility between the plating layer and the underlying steel sheet, which may be fundamentally a main factor contributing to the plating adhesion, has not been solved, and therefore, the plating (or coating) separation cannot be prevented from occurring during severe working.